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Cat 568A or B: Which is Right For HDMI?

The two cables may look alike, but they don't terminate the same. Here is what you need to know when using Category 568A and B.


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Someone asked if it was better to use Category 568A or 568B with HDMI. The short answer: neither. Any high-speed engineer learns early in his career that going direct and straight is the best. So why would manufacturers use a 568A or B wiring scheme?

You are the reason. They know that you are familiar with 568A or B and to use it for high-speed HDMI would require you to learn terminating RJ45s in sequential pairs, not 568.
       
So the bigger question is why are you trying to apply telephone and/or Ethernet applications to HDMI? My guess is that the wire looks the same, the connector looks the same, so why not terminate the same. That’s where the similarities end.
       
The connector that we typically use for Cat 5 ATD (balun) is an RJ45. This came from a family of connectors ranging from RJ11, RJ25, RJ61, etc. created to standardize telephone-related wires.

When introduced, the quality of the RJ connector was so impressive the FCC even incorporated it into its Part 68 rules. The RJ11 is a 6-pin device and can come in different arrangements for one, two or three analog telephone lines.

However, four analog voice lines used an 8-pin connector called the RJ61. The RJ45 term was essentially decided on by the EIA/TIA to be used for Ethernet, but it became the standard for both voice and data, and so came the 568 standard.

Here is where it gets interesting. Under 568A the two center pins 4 and 5 were isolated by splitting the orange pair to pin 3 and 6, trapping the blue pair on pin 4 and 5 and reversing its phase. The reason was to accommodate the voice telephone line should someone plug an RJ11 into a female RJ45 connector. Under 568B rules the only difference is the orange split pair is replaced by the green, and the orange is relocated where the green came from.


  About the Author

Jeff Boccaccio is president of DPL Labs. Jeff can be reached at [email protected] Have a suggestion or a topic you want to read more about? Email Jeff at [email protected]

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